1. Field of the Invention
The present invention relates to an organic semiconductor, a photoelectric conversion element, a solid-state image device in which an organic photoelectric conversion part is preferably formed on an electric charge accumulation/transfer/readout substrate, and a compound useful for the organic semiconductor.
2. Description of the Related Art
Conventional visible light sensors in general are a device fabricated by forming a photoelectric conversion site through, for example, formation of PN junction in a semiconductor such as Si. As for the solid-state image device, there is widely used a flat light-receiving device where photoelectric conversion sites are two-dimensionally arrayed in a semiconductor to form pixels and signals generated by photoelectric conversion in each pixel are charge-transferred and read out according to a CCD or CMOS format. The method for realizing a color solid-state image device is generally fabrication of a structure where on the light incident surface side of the flat image-receiving device, a color filter transmitting only light at a specific wavelength is disposed for color separation. In particular, a single-plate sensor in which color filters transmitting blue light, green light and red light, respectively, are regularly disposed on each of two-dimensionally arrayed pixels is well known as a system widely used at present in a digital camera and the like.
In this system, since the color filter transmits only light at a limited wavelength, untransmitted light is not utilized and the light utilization efficiency is bad. Also, in recent years, with the continuing progress toward fabrication of a multipixel device, the pixel size and in turn, the area of a photodiode part become small and this brings about problems of reduction in the aperture ratio and reduction in the light collection efficiency.
In order to solve these problems, there may be considered a system where photoelectric conversion parts capable of detecting light at different wavelengths are vertically stacked. As regards such a system, for example, U.S. Pat. No. 5,965,875 discloses a sensor utilizing wavelength dependency of the absorption coefficient of Si, where a vertically stacked structure is formed and the colors are separated by the difference in the depth, and JP-A-2003-332551 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) discloses a sensor having a stacked structure using an organic photoelectric conversion layer. However, the system utilizing the difference in the depth direction of Si is originally disadvantageous in that the color separation is poor, because the absorption range is overlapped among respective portions and the spectroscopic characteristics are bad. As for other methods to solve the problems, a structure where a photoelectric conversion film of amorphous silicon or an organic photoelectric conversion film is formed on a signal reading substrate is known as a technique for raising the aperture ratio.
Heretofore, several examples have been known for a photoelectric conversion element, an image device and a photosensor each using an organic photoelectric conversion film. It is a task in particular to achieve high photoelectric conversion efficiency and low dark current, and as to the improvement method in this respect, there are disclosed, for example, introduction of a pn-junction or introduction of a bulk-heterostructure for the former and introduction of a blocking layer for the latter.
These structural improvements have a large effect but the characteristics of the material used also greatly contribute to the device performance, which is referred to, for example, in JP-A-2006-086160 and JP-A-2006-100502. The material structure is not only one of main factors for the photoelectric conversion efficiency (exciton dissociation efficiency, charge transport property) and dark current (e.g., amount of dark time carrier) but also a governing factor for the signal responsivity, though this is scarcely mentioned in past reports. In use as a solid-state image device, all of high photoelectric conversion efficiency, low dark current and high response speed need to be satisfied, but it has not been heretofore specifically disclosed what an organic photoelectric conversion material or a device structure satisfies this requirement.
Also, JP-A-54-024025 describes use of a 2,4-chromandione derivative as an electrically photosensitive colorant for use in an electrophoretic migration imaging process, but use of the compound for an organic photoelectric conversion film is not described.